1. Technical Field
The present invention relates to a fixing device which is used in formation of an electrophotographic image and performs fixing by a heating belt, and an image forming apparatus including the same.
2. Related Art
A belt-fixing type fixing device is known as a fixing device of an image forming apparatus. This belt-fixing type fixing device includes an endless heating belt and a pressurization roller. The heating belt is stretched over a fixing roller and a heating roller with tension. A toner image of a transfer material is heated and pressurized at a nip portion (pressing portion) of the heating belt and the pressurization roller so as to be fixed on the transfer material.
However, in the belt-fixing type fixing device, a fixing belt stretched over a pair of rollers may be skewed and thus the fixing belt may be damaged. In order to prevent this, a fixing device which suppresses the bias of the fixing belt by guide rings provided at both ends of the heating roller so as to prevent the skew of the fixing belt is suggested in Japanese Patent No. 3711717, for example. In the fixing device, a vertical surface and an inclined surface are provided in the wall surfaces of the guide rings facing the fixing belt. By the vertical surfaces of the guide rings, the bias of the fixing belt is suppressed. In addition, when the fixing belt rides over the inclined surface by the skew, the fixing belt is prevented from riding over the inclined surface.
Generally, the heating roller is formed of, for example, a metal material such as aluminum, in view of strength and heat transfer to the heating belt. In addition, the heating belt is formed of a thin flexible belt obtained by coating a belt base material such as nickel with silicon rubber or the like. Accordingly, a thermal expansion coefficient of the heating roller is relatively large and a thermal expansion coefficient of the heating belt is relatively small. In addition, when a power source of the heating roller is turned off, the belt has a low temperature. However, when the power source of the image forming apparatus is switched from an OFF state to an ON state or an image forming command is generated from a sleep mode (power saving mode), a belt heating operation and a belt rotation operation are substantially simultaneously started.
However, when the image forming apparatus is set to the above-described mode after the heating belt is heated by an image forming operation, the heating belt and the heating roller are cooled. At this time, a heat contraction amount of the heating roller in an axial direction becomes larger than that of the heating belt in a width direction. Then, as shown in FIG. 14, a guide ring b of a heating roller a strongly presses a belt edge d of a heating belt c with force Ft. In this case, a central portion of a width direction of the heating belt c cannot easily slide in the width direction due to friction with the heating roller (not shown). Accordingly, large stress is applied to an end (an area of the belt close to the belt edge in the width direction) of the heating belt c and several wrinkles e (waving of irregularities) are generated in the heating belt c in the width direction of the belt.
If an image forming command is generated in a state in which the wrinkles e are generated, the belt heating operation and the belt rotation operation are substantially simultaneously started as described above. Then, since the edge d of the belt portion entering the heating roller a is continuously pressed by the guide ring b in the width direction and the wrinkles e are moved, adjacent concave portions overlap with each other and crack occurs in the heating belt c.